Low-loss, high-speed, high-Tc superconducting bearings

ABSTRACT

A flywheel energy storage device including an iron structure disposed for rotation adjacent a stationary superconductor material structure and a stationary permanent magnet. The stationary permanent magnet levitates the iron structure while the superconductor structure can stabilize the rotating iron structure.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has certain rights in this inventionpursuant to Contract No. W-31-109-ENG-38 between the U.S. Department ofEnergy and The University of Chicago, representing Argonne NationalLaboratory.

This is a continuation of application Ser. No. 08/025,950 filed on Mar.3, 1993, now U.S. Pat. No. 5,540,116.

BACKGROUND OF THE INVENTION

The present invention generally relates to a high temperaturesuperconductor ("HTSC") device. More particularly, the invention relatesto structural geometries for rotational HTSC devices to achieve lowerrotational dissipations, higher magnetic pressures and higher rotationalvelocities.

Efficient electrical energy storage is useful in numerous practicalapplications. Diurnal storage of electricity is important to electricutilities in order to efficiently utilize base load generating plantsand to meet the varying load demands of their customers. Base loadplants can charge storage units at night when demand is low, and thenpeak demands can be met by discharging the storage units during the peakhours.

Energy storage can also play a substantial role in eliminating orpostponing the installation of larger capacity power lines. For example,power can be transferred from a baseload generating plant to asubstation having energy storage units at night when demand is low.During peak power demand times, the energy storage units can bedischarged. These energy storage units can also be located in otherparts of the electrical distribution system: utility parks where largeamounts of energy can be stored; in tandem with photovoltaic or windenergy generation facilities that are time dependent; substation units;and individual companies and homes. Similar energy storage units can beused on electric vehicles such as cars and buses, or as wayside energystorage for electric trains.

Flywheels are often considered for energy storage unit applications.Their primary advantages are modularity, high energy storage density(Wh/kg), and high efficiency input and output of electrical energy. Theability to produce high strength flywheel rotors and the ability toefficiently transfer energy in and out of a flywheel are well known andwill not need to be discussed in this application.

The primary disadvantage of conventional flywheels is inefficiency inthe standby mode. Substantial energy losses occur because theconventional ball beatings that support the flywheel structure have highlosses. Conventional ball beatings have relatively large coefficients offriction, are subject to wear and also require lubrication.

Alternatively, magnetic beatings can be used to support the flywheelstructure. Conventional magnetic beatings have no contacting parts,require no lubrication, and often have lower losses than ball beatings.However, magnetic beatings require position sensors and feedbackelectronics to keep the beatings stable. Further, energy lossesassociated with conventional magnetic beatings are still sufficientlyhigh that diurnal storage of energy in a flywheel is relativelyinefficient. Energy losses in a conventional magnetic beating areattributable to magnetic drag and parasitic losses from passing currentthrough the windings of the electromagnets in the beatings. Astate-of-the-art flywheel energy storage unit with a magnetic beatingtypically loses about 1% of the stored energy per hour due to energylosses attributable to the beatings.

Superconducting beatings have the potential to reduce such energy lossto very low values. One example of such a structure is described in U.S.patent application Ser. No. 07/736,677, which is incorporated byreference herein in its entirety, and is assigned to the owner of theinstant invention. In this other application, a permanent magnet isrotated over an HTSC material structure. The disadvantage of thisconfiguration is that any azimuthal nonuniformity in the rotatingpermanent magnets produces an alternating current magnetic field at thesurface of the HTSC material. This field, in turn, induces hysteresislosses in the HTSC material; and the associated energy must be removedat cryogenic temperatures. Another disadvantage of this type of beatingis that the permanent magnet has a relatively low tensile strength andtherefore cannot withstand high rotational speeds. In addition, there isa limitation in magnetic pressure due to the magnetic fields of thepermanent magnets.

It is therefore an object of the invention to provide a novel low-lossmagnetic beating and method of use in a high-efficiency flywheel energystorage device.

It is a further object of the invention to provide an improved flywheelwith low standby energy losses when the storage time is about one day orlonger.

It is yet another object of the invention to provide a novel low dragforce magnetic beating comprising a permanent magnet, an HTSC materialand a ferromagnetic rotor.

It is still a further object of the invention to provide an improvedrotor capable of very high rotational speeds using a superconductormaterial beating.

It is an additional object of the invention to provide a novelhigh-efficiency flywheel energy storage device with a high degree ofrotational symmetry.

It is yet a further object of the invention to provide an improvedhigh-efficiency flywheel energy storage device with high magnetic fieldazimuthal homogeneity.

It is still an additional object of the invention to provide a novelhigh-efficiency flywheel energy storage device having lower rotationaldissipation and higher magnetic levitation pressure.

It is yet another object of the invention to provide an improvedflywheel energy storage device which minimizes hysteresis and eddycurrent losses.

Other objects, features and advantages of the present invention will bereadily apparent from the following description of the preferredembodiments thereof, taken in conjunction with the accompanying drawingsdescribed below wherein like elements have like numerals throughout theseveral views.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a cross-sectional view of a flywheel energy storagedevice constructed in accordance with one form of the invention whichincludes a levitated iron ring and steel flywheel; FIG. 1B shows across-sectional view of an alternative embodiment of the device shown inFIG. 1A and having a steel flywheel and iron ring comprising a series ofnested bands; FIG. 1C illustrates a cross-sectional view of analternative embodiment of the device shown in FIG. 1A and having aflywheel and iron ring comprising a series of nested steel bandssurrounded by fiber-composite rings;

FIG. 2 shows a cross-sectional view of an alternative embodiment of aflywheel energy storage device including a levitated flywheel and aniron ring disposed within a stationary permanent magnet and HTSCmaterial rings;

FIG. 3A illustrates a cross-sectional view of a stationary ironstructure supporting a permanent magnet and a stationary HTSC ringlevitating an iron flywheel; FIG. 3B shows a cross-sectional view of aflywheel with an embedded iron ring levitated within an enclosed ironstructure; FIG. 3C illustrates a cross-sectional view of a flywheelhaving an embedded iron ring levitated about the circumference of thebeating; FIG. 3D shows a cross-sectional view of a levitated flywheelincluding an iron center portion; and FIG. 3E illustrates across-sectional view of another embodiment of one form of the inventionwherein an HTSC materrial-encapsulated iron structure includes permanentmagnets for levitating an iron flywheel ring;

FIG. 4A shows a cross-sectional view of a permanent magnet ringlevitated by a stationary HTSC material ring and a stationary magnetwhich can be adjustably positioned; FIG. 4B illustrates across-sectional view of an alternative stationary magnet for thestructure shown in FIG. 4A wherein the stationary magnet for positionadjustment comprises an iron ring with a radially magnetized permanentmagnet disk at its center;

FIG. 5A shows a cross-sectional view of a permanent magnet flywheeldisposed within an iron cup levitater by HTSC material and asupplementary magnet; FIG. 5B illustrates a cross-sectional view of analternative flywheel magnet structure disposed in an iron cup includingradial bands and several magnetic rings for use in the structure shownin FIG. 5A; and FIG. 5C shows a cross-sectional view of an alternativecup magnet structure for the embodiment shown in FIG. 5A; and

FIG. 6 illustrates a permanent magnet flywheel consisting of a series ofalternating polarity magnets.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the figures and more particularly to FIG. 1A, a flywheelenergy storage device (hereinafter "flywheel device") constructed inaccordance with one form of the invention is indicated generally at 10.This preferred embodiment incorporates magnetic beatings 12 that utilizea permanent magnet disk 14 and an HTSC material ring 16 to levitate asteel flywheel rotor 18 connected to an iron ring 20. The iron ring 20interacts magnetically with the stationary permanent magnet disk 14 toprovide stable vertical levitation. The HTSC material ring 16 isinterposed between the iron ring 20 and the permanent magnet disk 14.When magnetic flux passes through, and is pinned in the HTSC materialring 16, the magnetic field provides horizontal stability to the steelflywheel rotor 18.

In the preferred embodiments described herein, as a nonlimiting example,"Type II" HTSC materials have been found to yield excellent results.These materials enable some magnetic flux to penetrate into the interiorof the material itself in clusters of flux lines. This allows Type IIHTSC material to generate large magnetic fields and exert a "pinning"effect on a rotating magnetic flywheel. Further information on thesematerials can be found in U.S. Pat. No. 4,939,120 (Moon, et al.) whichis incorporated by reference herein in its entirety. The HTSC materialmust be cooled to a temperature below T_(c) to become superconducting.This cooling can be accomplished by immersing the HTSC material in acryogenic fluid, such as liquid nitrogen, or by thermally connecting theHTSC material to a refrigerator component via a conductive coldfinger.Because these techniques are conventionally known, they are typicallynot shown in the figures. It will be further understood by one skilledin the art that the flywheel device 10 can be encased in a vacuumchamber and will have a means of providing power input and output.

In the preferred embodiments of the invention wherein the permanentmagnets are not part of the flywheel rotor 18, the iron ring 20 ispreferably mechanically constrained before and during, but not after,the field cooling of the HTSC material ring 16. While ring-shapedstructures are described herein for nonlimiting, illustrative purposes,other shapes may be used equivalently in the embodiments describedherein. It will be apparent to one skilled in the art that the iron ring20 can be composed of any material with a high magnetic permeability(e.g., magnetic steels or nickel and its alloys) and is preferably ofhigh mechanical strength.

Further, the steel flywheel rotor 18 and/or the iron ring 20 shown inFIG. 1A (and for all subsequent figures and embodiments) need not be asingle ring. As shown in FIG. 1B, the steel flywheel rotor 18 and/or thering 24 can instead comprise a series of nested steel bands 22 with theouter ones of the bands 22 putting the inner ones of the bands 22 incompression.

An alternative embodiment of this nested steel band concept surroundsthe steel ring 24 (or series of bands 22) by a light-weightfiber-composite ring 26 that puts the steel ring 24 in precompression asshown in FIG. 1C. The fiber composite ring 26 composite can be made, forexample, from E-glass epoxy, Kevlar®, a trademark of E. I. Du Pont deNemours and Company for a durable spun fiber composite material,carbon-carbon composite, or other equivalent materials. Theprecompression helps offset the tension which results from centrifugalforces, thereby allowing rotation of the steel ring 24 at higherrotational velocities without structural failure.

In another embodiment of the invention illustrated in FIG. 2, thepermanent magnet 28 comprises a stationary rare earth magnet ring 30located outside of the iron ring 20. A stationary HTSC material ring 16is located between the iron ring 20 and the rare-earth magnet ring 30and again provides horizontal stability.

As shown in FIG. 3A, an alternative embodiment of one form of theinvention includes two stationary permanent magnets 32, an iron flywheelring 34 and an iron yoke 36 which form an efficient magnetic circuit.HTSC material disks 38 are placed between the iron flywheel ring 34 andeach stationary permanent magnet 32. In this configuration, the magneticcircuit provides a large magnetic flux density and stable lateralpositioning of the iron flywheel ring 34. The flux is trapped in thefield-cooled HTSC material disks 38 which provide both verticallevitation and stability. The top and bottom gaps between the stationarypermanent magnets 32 and the iron ring 34 can be of different dimensionsto provide varying vertical levitation. Alternatively, a permanentmagnet ring 40, HTSC material ring 16 and iron shaft 42 can be used inthe same arrangement shown in FIG. 3A. Various geometries using thisconcept are illustrated in FIGS. 3B, 3C and 3D. For clarity, theindividual HTSC material and permanent magnet elements are not shown,but are similar in arrangement to FIG. 3A.

The configuration of FIG. 3E is a generalization of the basic conceptshown in FIG. 3A, wherein some efficiency of the magnetic circuit issacrificed to achieve larger levitation forces. An iron flywheel disk 44is levitated by stationary rare earth permanent magnets 46 which areencased in a stationary iron disk 48. The stationary iron disk 48 isencapsulated by an HTSC material structure 50. Parameters that can bevaried to achieve a wide range of design objectives include, forexample, the following: (1) the distribution of the stationary rareearth magnets 46; (2) the shape of the stationary iron disk 48 (forcompleting the magnetic circuit); (3) the spacings between thestationary iron disk 48 and the levitated iron flywheel disk 44; and (4)the variable thickness HTSC material structure 50 encapsulation.

FIG. 4A illustrates a configuration in which the magnetic pressure isincreased over that amount which can be obtained with only a permanentmagnet ring 40 being levitated over an HTSC material ring 16. Agenerally cylindrical liquid nitrogen "coldfinger" 52 or cryogenimmersion surrounds a stationary cylindrical magnet 54 in thisembodiment. The stationary cylindrical magnet 54 is placed on aninsulator 56 of selectable height and position. A stationary HTSCmaterial ring 16 preferably comprising HTSC material pieces is placed ontop of, and is substantially surrounded by, the liquid nitrogencoldfinger 52. The rare earth permanent magnet ring 40 is levitated overthe stationary HTSC material ring 16 and the stationary cylindricalmagnet 54. In this embodiment, the stationary cylindrical magnet 54provides an additional levitation force on the rare earth permanentmagnet ring 40. Therefore, because the amount of rotor weight able to besupported increases for the same rotating magnetic field on the surfaceof the HTSC material ring 16, this configuration should provide lowerlosses than if the stationary cylindrical magnet 54 were absent. Also,this embodiment provides greater control of the positioning of thepermanent magnet ring 40. This is accomplished by allowing the positionof the stationary cylindrical magnet 54 to be adjusted to help balancethe forces in the system.

FIG. 4B shows an alternative embodiment of the stationary magnetconfiguration illustrated in FIG. 4A. In this embodiment, the stationarymagnet structure comprises a permanent magnet disk 14 generallysurrounded by a stationary iron ring 58. The permanent magnet disk 14 ismagnetized radially. The stationary iron ring 58 serves to decreases thereluctance in the magnetic circuit, thereby further increasing thelevitation force.

FIG. 5A shows an apparatus in which a large levitation force isachieved, and a rare earth rotor magnet 60 can also achieve highrotational velocities. In this embodiment, a supplementary magnet 62 islocated below the generally cylindrical liquid nitrogen coldfinger 52.An HTSC material disk 38 (or plurality of HTSC material pieces) isgenerally contained within the liquid nitrogen coldfinger 52. An ironcup 64 with the rare earth rotor magnet 60 located therein is levitatedabove the HTSC material disk 38 and the supplementary magnet 62. Thesupplementary magnet 62 increases the magnetic levitation force byacting directly on the rare earth rotor magnet 60. The HTSC materialdisk 38 also produces a levitation force, as well as providinghorizontal stabilization. The iron cup 64 positioned over the rare earthrotor magnet 60 decreases the reluctance of the magnetic circuit aroundthe rare earth rotor magnet 60, thus increasing the magnetic field andlevitational force. It also increases the gradient of the magneticfields near the boundary between the rare earth rotor magnet 60 and theiron cup 64, thus increasing the stiffness of the levitational force.Additionally, the iron cup 64 can precompress the rare earth rotormagnet 60 and act as a strengthening material to keep the rare earthrotor magnet 60 intact at high rotational speeds. Finally, small magnetsor pieces of iron can be added or iron pieces can be removed from theiron cup 64 to improve the azimuthal homogeneity of the magnetic field.

In the alternative embodiment shown in FIG. 5B, the rare earth rotormagnet 60 comprises a series of concentric permanent magnet rings 66separated by concentric iron rings 68. The iron rings 68 act both asstrengthening members and as reluctance reducers. The magnet rings 66can be selectively rotated to achieve better magnetic field axialsymmetry. Additionally, the moments of the magnet rings 66 can bealternated to provide larger magnetic gradients and levitationstiffness.

Yet another alternative embodiment is illustrated in FIG. 5C, in whichthe rotor magnet 60 comprises a series of disks 72 stacked on top ofeach other. Asymmetric magnetic fields are often produced by rare earthmagnets due to imperfections in the magnet structure. By appropriatelyrotating each of the disks 72 relative to the others, asymmetries in themagnetic field produced by the rotor magnet 60 can be reduced. Forexample, with two of the disks 72, the maximum field of one of the disks72 can be located directly under the minimum field of the second. Theasymmetry of the combined field will be the lowest possible for thesetwo disks 72.

Another embodiment of the configuration of FIG. 5A is shown in FIG. 6wherein the rare earth rotor magnet 60 comprises a series of alternatingpolarity magnet rings 74. These magnet rings 74 increase the stiffnessof the levitational force by channeling the magnetic flux betweenadjacent poles of the magnet rings 74. The iron cup 64 surrounding theentire assembly acts to mechanically strengthen the rare earth rotormagnet 60 and reduce the reluctance of the magnetic circuit.

Accordingly, the preferred embodiments described herein significantlydecrease energy losses attributable to beatings in a flywheel energystorage device. Practical applications for which these devices havehistorically been unsuitable can now utilize the significant efficiencyand performance advantages of the present invention.

While preferred embodiments have been illustrated and described, itshould be understood that changes and modifications can be made thereinwithout departing from the invention in its broader aspects. Variousfeatures of the invention are defined in the following claims.

What is claimed is:
 1. A flywheel energy storage device, comprising:arotational iron structure disposed for rotation adjacent a stationarysuperconductor material structure and a stationary permanent magnetstructure, said iron structure being levitated by said stationarypermanent magnet structure.
 2. The device of claim 1, wherein saidstationary superconductor material structure and said stationarypermanent magnet structure are disposed within at least a portion ofsaid iron structure.
 3. The device of claim 1, wherein said ironstructure comprises ferromagnetic steel.
 4. The device of claim 1,wherein said iron structure comprises a plurality of steel bands.
 5. Thedevice of claim 1, whereto at least a portion of said iron structure issubstantially surrounded by a composite material structure.
 6. Thedevice of claim 5, wherein said composite material structure is disposedto produce compressive forces in said iron structure.
 7. The device ofclaim 1, wherein said stationary high temperature superconductormaterial structure comprises a plurality of substantially concentricrings.
 8. The device of claim 1, wherein said stationary permanentmagnet structure comprises a plurality of substantially concentricrings.
 9. The device of claim 1, wherein said iron structure comprises adisk-shaped structure.
 10. The device of claim 1, wherein said ironstructure comprises a ring-shaped structure.
 11. The device of claim 1,wherein said iron structure comprises a substantially cylindrical-shapedstructure.
 12. The device of claim 1, wherein said stationarysuperconductor material structure comprises a superconductor materialcapable of allowing magnetic flux to penetrate said material.
 13. Thedevice of claim 1, wherein said stationary superconductor materialstructure is disposed between said iron structure and said stationarypermanent magnet structure.
 14. The device of claim 1, further includinga second stationary high temperature superconductor material structureand a second stationary permanent magnet structure for producingadditional levitational pressure upon said iron structure.